The present invention relates to batch annealing apparatuses, and in particular to bell annealing apparatuses and processes in which annealable materials are annealed in a controlled atmosphere.
The annealing process subjects an alloy, metal, glass, special polymer, or the like to a process of heating and cooling to make it tougher, less brittle, more resistant to fracture, and the like, and/or to depart some special characteristic(s) thereto. Annealing minimizes internal defects in the atomic structure of the material and leaves it free from internal stresses that might otherwise be present because of prior processing steps. Annealing can be used to restore the ductility of work hardened metals and alloys, and to depart particular electrical characteristics on circuit components. Numerous types of annealing materials are known in the art.
Ferrous and nonferrous metals, including carbon/stainless steel and exotic alloys, and glass are annealed by heating them to high temperatures and cooling them slowly; copper and silver, however, are best annealed by heating and cooling quickly, then immersing in water. Batch annealing apparatuses and processes have been used for many years to anneal coils of materials such as coils of steel strip, wire, or rod, and to anneal panels, discrete objects, such as microchips (or processors), glass, and the like. Metallic coil annealing practices, for example, are used in the manufacture of punching quality oriented silicon steel, regular grain oriented silicon steel, high permeability oriented silicon steel, and all types/alloys of stainless steel.
The external dimensions of bell annealing furnaces vary greatly. Bell annealing furnaces, which are also known as box annealing furnaces, are used in industrial batch annealing processes, in both small and large scale applications. Large masses of metal or glass are cooled within industrial sized annealing furnaces in which cranes, or the like, are used to lift the furnace off of a base which holds the annealable materials thereon. Table top annealing furnaces may be used in academic settings and for small scale manufacturing purposes, and may have doors in the side which enclosed an opening within the furnace that holds the annealable materials therein. Some large scale annealing furnaces may also have a door in the side which enclosed an opening within the furnace that holds the annealable materials therein.
Annealing time, especially of glass, varies widely according to the thickness of the individual piece. Window glass, for example, requires several hours; plate glass, several days; and glass mirrors for reflecting telescopes, several months. Annealing is required as an intermediate step in metal-forming processes such as wire drawing or brass stamping in order to restore the ductility of the metal lost because of work hardening during the forming operation. Other applications are also well known in the art.
At minimum, a bell annealing furnace includes an outer furnace structure or cover, and an inner cover structure disposed within the outer furnace structure. A heating means is disposed within the furnace to heat the space between inner and outer structures, or more specifically, to heat the external surface of the inner cover structure. Bell type annealing furnaces may be direct-fired with combustible fluid fuel or heated by an electrical heating means such as sinuously-shaped resistor ribbons. Either inner or outer structure may be cylindrical, rectangular, or square in shape, as long as the outer structure encompasses the inner structure. In operation, the atmospheres within both inner and outer structures are closed to each other, and to the outside. Means are provided to control the atmosphere in both inner and outer structures. A sealable opening may be provided for access to a space within the inner cover of all bell annealing furnaces to facilitate the addition and removal of annealable substrate materials, or the inner and outer covers may simply be removed.
In some annealing furnaces, a vacuum can be drawn, and/or inert gasses may be provided within the inner structure and/or the outer structure depending on the annealing process desired. Apparatus may also be provided to hold the substrate materials within the inner structure. A base is typically provided in many embodiments to which both inner and outer covers are sealed. In these embodiments, the inner and/or outer covers are removable by lifting the bell furnace off of the base, for example, using a crane. Alternative embodiments provide for a sealable door in the side of the annealing furnace whether rectangular or cylindrical in shape. In the case of cylindrical shaped furnaces, the axis may be disposed parallel to the ground with an opening through a round end of the cylindrical furnace, the other end being closed.
A typical annealing furnace includes an outer furnace structure, a base which provides a support for the materials to be annealed, such as coils, and a bell cover which is placed over the coils within the furnace and sealed against the base or against the encompassing walls of the outer furnace to provide a sealed interior for circulation of a controlled atmosphere, such as an inert gas, as required for annealing or other processes. Hydrogen and nitrogen are considered inert gasses in these processes. A means, such as a fan, for circulating the gas within the bell cover is typically provided on, or in fluid communication through, the base. The encompassing outer furnace is heated either by electricity or gas combustion and is scaled over the bell cover sometimes sealing on the bell cover, and sometimes sealing on the base depending on configuration of the particular bell furnace. Alternative annealing structures provide for a plurality of inner covers to be disposed therein, or for a sealable door to replace the base.
In the manufacture of unidirectional electrical steel, batch annealing furnaces are in wide use for finish annealing (secondary recrystallization annealing). In these examples, a coil of electrical steel is placed, with the axis thereof vertical, on a base plate. An inner cover is placed over the coil, and a bell-shaped furnace lowered over the inner cover. An atmospheric gas, such as N2, Ar, H2 or the like, is supplied into the space under the inner cover through a feed pipe. At least one electric heater, or at least one gas burner, is disposed on the inside of the furnace to heat the gas/air between the inner wall of the furnace and the outer wall of the inner cover. An apparatus is provided to cool the heated air/gas in the furnace once the coil has been heated to a given temperature by communicating cool air/gas through the furnace between the inner wall of the furnace and the outer wall of the inner cover. This typically involves a cooling gas supply pipe connected to the top of the furnace and an exhaust of the heated cooling gas through a cooling device which then recirculates the cooled cooling gas through the furnace again.
In some embodiments of a bell annealing furnace, the furnaces are lined with refractory bricks, usually outer furnace, which may be tied together, affixed with mortar, or otherwise conventionally attached to the walls of the furnace or bell. Unfortunately, conventional refractory brick lining absorbs significant amounts of heat, lengthening the heat-up portion of the furnace cycle. Furthermore, conventional refractory bricks have high heat retention properties that tend to prolong the cool-down portion of the furnace cycle. Alternatives to refractory bricks include sheets, or panels of refractory materials, as well as other well known refractory materials. The base or door may also be lined with, or composed of, refractory materials.
There are significant temperature gradients between different locations of an annealing furnace structure. It is desirable to promote maximum air flow and circulation to prevent any hot spots or heat build-ups at elevated temperatures. To this end, attention has been given to improved seals between the outer furnace and the bell cover, and the base of the furnace, and improved atmosphere flow controls throughout. Additionally, many annealing processes require specific heat gradients to be maintained prior to a uniform cooling process. Alternative, conventional metal coil annealing furnaces may anneal a stationary coil placed on a base plate by using a given heat pattern to obtain a desired result.
An example of a conventional bell annealing furnace is shown in U.S. Pat. No. 2,283,982 assigned to Westinghouse Electric & Manufacturing Company on May 26, 1942, which teaches a bell type annealing furnace having a refractory-metal inner hood placed about a charge, the hood being encased by an outer insulating bell with sufficient clearance space between the hood and the interior of the bell to permit the disposition of heating means therein. This bell annealing furnace has a cylindrical formed hood and bell to aid in the even distribution of heat throughout the space between hood and bell.
An example of an annealing furnace used with a metal coil is shown in U.S. Pat. No. 4,502,671, which was assigned to Nippon Steel Corporation, issued on Mar. 5, 1985, and teaches another batch annealing apparatus consisting of a bell-shaped furnace, a base plate, and an inner cover. The apparatus therein also has a bottom chamber that opens upward, which has a diameter slightly larger than the outside diameter of a metal coil to be treated therein.
Various methods are used to seal the base or door, and to support the coils or other substrate to be annealed. U.S. Pat. No. 6,063,331, assigned to Rad-Con Incorporated and issued on May 16, 2000, teaches a bell type annealing furnace structure which includes a base with a base plate member and an annular channel therearound. A plurality of enclosure members having a coil support section with support legs depending therefrom supporting the enclosure member on the base plate member and defining a space between the base plate member and the coil support section. An improved seal arrangement is contemplated in which the seal uses a wire mesh with enclosed ceramic fibers.
U.S. Pat. No. 4,504,957 issued on Mar. 12, 1985 and assigned to Armco Inc. teaches a high temperature box annealing furnace for metallic coil annealing practices. That furnace has a fixed base and a removal bell capable of achieving a sealed relationship with the base. The interior of the bell side walls, end walls and roof are lined with ceramic fiber insulation and are provided with electrical resistance heating elements. The base has a steel framework supporting a cast refractory base member configured to support one or more coils. Each coil is provided with a cover and the cast refractory member provides a sand, water and/or sand seal for the lower edge of the covers. Multiple inner covers are used simultaneously within the same furnace. Cooling capacity is provided by inlets and outlets for appropriate cooling gas. The base is ceramic eliminating the metallic coil-supporting base plates of other furnaces. Furthermore, fiber insulation is provided within the bell in an effort to reduce heat up and cool down portions of the furnace cycle.